This invention relates to systems and methods for adjusting cable modem power and frequency levels when noise is present in the cable network. More specifically, the invention relates to cable network systems that calculate power and/or frequency adjustments based upon an average (or some other statistical measure) of recently measured power and frequency levels.
In cable network systems (e.g., hybrid fiber-coaxial (or HFC) plants), digital data is carried over radio frequency (RF) carrier signals. At the interfaces of a cable network are cable modems. These devices modulate digital data for “upstream” transmission on a broadband media and demodulate modulated RF signals for “downstream” reception of digital data.
Most cable networks are designed so that the head-end (or the cable modem termination system (CMTS) component of the head-end) receives communications from cable modems at some baseline power level (e.g., 0 decibels). The CMTS, and hence the cable network, will not perform properly if upstream signals from cable modems deviate significantly from this expected baseline power. Due to cable network topology, different cable modems must send upstream signals at different power levels. This situation may be better understood with reference to FIG. 1.
FIG. 1 is a block diagram of the upstream components of a two-way HFC cable system 101. The system includes various cable modems and a CMTS 104. Note that the depicted system includes three separate distribution networks 121A-C connected to three different upstream ports of CMTS 104.
The described HFC cable system may be used for two-way transmission of digital data such as Internet data, digital audio, or digital video data in MPEG format, for example. The data arrives from one or more external sources (not shown) through CMTS 104. The CMTS converts the digital data from these sources to a modulated RF signal that is carried over the fiber and coaxial lines to the subscriber premises. The cable modems demodulate the RF signal and feed the digital data to computer their respective computers. On the return path, the operations are reversed. The digital data is fed to the cable modem, which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
The main distribution component of HFC cable system 101 is a hub 102 (also referred to as the “head-end”) which can typically service about 40,000 subscribers or end-users. Hub 102 contains several components, most notably CMTS 104. From CMTS 104, separate downstream and upstream lines are used for sending and receiving data. Downstream signals pass through an upconverter 123 and onto other components on their way to destination cable modems. Upconverter 123 converts the frequency at which downstream signals are carried. All data from CMTS 104 is carried in an “intermediate frequency” that is independent of both channel and service. Upconverter 123 coverts intermediate frequency signals channel specific radio frequency signals. The downstream RF signals are broadcast over the cable plant and ultimately received at the cable modems. This figure shows cable modems as blocks 105, 106, 107A-C, 108, 108, 109, 111, 112, and 113. The components on the downstream path between upconverter 123 and the cable modems are not depicted.
On the upstream path, data from some of the cable modems enters cable plant 101 via distribution networks 121A-C. Each distribution system will have its own topology, which varies as a function of the local neighborhood geography, the number and type of cable modems in the distribution network, etc. As a result, signals from different modems on a given distribution network are attenuated by different amounts on the upstream path.
Data from modems on the distribution networks is typically transmitted as electrical signals over conventional coaxial cables 120A-C, also referred to as a trunk lines. In some instances, coaxial trunk lines are replaced with optical fiber. Data traveling upstream from trunk lines 120A-C reaches fiber nodes 118A-C, which convert the electrical signals to optical signals that can be transmitted over fiber optic cables 116A-C. Typically fiber optic cables 116A-C contains pairs of cables carrying data in opposite directions. These cables typically run for as long as 100 km and are used to carry data for most of the distance between the neighborhood distribution networks and hub 102.
Hub 102 can typically support up to 80 fiber nodes and each fiber node can support up to 500 or more subscribers. Thus, there are normally multiple fiber optic cables emanating from hub 102 (only three are shown to simplify the illustration). Note that in many systems, a technology know as dense wave-division multiplexing (DWDM) increases fiber capacity (and consequently the number of potential subscribers). DWDM is a technique for transmitting data via more than one wavelength of light on the same fiber.
Data from fiber optic cables 116A-C enters hub 102 via fiber transceivers 114A-C, which convert optical signals to electrical signals for processing in hub 102. Focusing on signals generated in the top distribution network 121A, upstream data from optical transceiver 114A passes through attenuators 125A and 127A before passing through a splitter 129A and onto a port of CMTS 104. As shown, parallel paths exist for distribution networks 121B and C.
To allow verification of the downstream path, hub 102 includes diplex filters 131A-C, which are connected to the downstream path (not shown) and to attenuators 133A-C. Cable modems 107A-C serve as part of the verification system. They are attached to diplex filters 131A-C to allow the CMTS to confirm that a modem can actually transmit on the associated distribution network. Thus, the network subsystem given by diplex filters 131A-C and cable modems 107A-C assist in verifying the operation of cable plant 101 and in isolating problems that arise in that plant. As shown, upstream data from the modems passes through one of diplex filters 131A-C and then onto one of attenuators 133A-C before entering one of splitters 129A-C.
As noted, the distribution networks have various topologies. Topological differences in paths from various modems to hub 102 result in different attenuation levels. Yet, CMTSs are designed to work at single power level. For example, CMTS 104 may be designed to receive signals centered at 0 dBmV amplitude. If all modems transmitted at the same power level, some signals received at CMTS 104 would be well above the required power level and some would be well below that level. To account for the effects of topological variations in the distribution networks, many cable systems require that different modems transmit at different power levels. The particular transmission power levels are selected to cause all signals to arrive at the CMTS with the same power level. The DOCSIS standard for transmission defines a procedure for adjusting modem transmission power such that the received power at the head-end is constant. This procedure, called “ranging,” is described below. Note that DOCSIS is an interim standard establishing the protocol for two-way communication of digital data on cable systems defined and adopted by a consortium of industry groups, and is widely-followed in the field of cable modem data communication.
FIG. 1 illustrates a hypothetical set of modem power levels chosen such that transmissions from all modems will reach CMTS 104 centered at 0 dBmV. As shown, transmissions from modem 105 are made at +31 dBmV and those from modem 106 are made at +48 dBmV. This difference results from different attenuation levels on the paths from these modems through distribution network 121A. Signals from modem 105 are less attenuated by the distribution network than are signals from modem 106. In the end, all transmissions, regardless of source modem, reach optical transmitter 118A at +17 dBmV. Similar results are illustrated for distribution networks 121B and 121C and their associated cable modems.
Because signals from all modems on a given distribution network experience the same attenuation while passing through the fiber optic and hub sections of the network, no further modem specific adjustments are required. As shown in FIG. 1, transmissions passing through optical components 118A, 116A, and 114A lose 4 dBmV so that they enter hub 102 as +13 dBmV electrical signals. Other optical sections have different effects. For example, optical components 118B, 116B, and 114B together amplify signals by 1 dBmV and optical components 118C, 116C, and 114C together amplify signals by 14 dBmV. Because the different fiber optic segments attenuate/amplify by differing amounts, hub attenuators 125A-C attenuate by different amounts so that all transmissions, regardless of source modem or fiber node, reach the CMTS at the same power level. Thus, all signals upstream from attenuators 125A-C should have the same power level (at the same position vis-à-vis CMTS 104). As shown, all transmissions leave attenuators 125A-C at +10 dBmV.
A hub can measure all signals at an “X” point. This point should be chosen at a location where all transmissions are expected to have the same power level with respect to the CMTS—regardless of fiber node or source modem. More precisely, the X point should be chosen, with respect to a CMTS amplitude detector location, so that power level is linearly related in a known manner that is consistent across all nodes feeding the CMTS. Usually, the X point will be at or proximate the CMTS. One common location is on a line card in the CMTS. In FIG. 1, the X point (indicated by reference numbers 135A-C) for each CMTS input port is located between attenuators 125A-C and attenuators 127A-C, respectively. At these points, the power level is expected to be +10 dBmV. Amplitude detectors may be positioned at these locations (or in the CMTS) so that they can measure power levels during testing. Note that in FIG. 1, the X points could be chosen to be any locations upstream of attenuators 125A-C.
The cable network shown in FIG. 1 is not static. Cable networks are composed of passive and active devices, each having a particular attenuation and frequency response. The overall frequency and amplitude response of a cable network system varies nearly continually. Cable lines are installed in new geographic areas, new cable modems and components are installed on existing segments of the network, the condition of existing lines deteriorates or improves, the performance of amplifiers, splitters, etc. changes, and so on.
As the power level of upstream signals changes, the CMTS attempts to cause the cable modems to adjust their output power levels so that it constantly receives upstream signals at 0 dBmV (or some other specified level). This adjustment may be accomplished via DOCSIS ranging messages. The DOCSIS ranging protocol for adjusting cable modem power and frequency is illustrated in FIG. 2 described below.
This approach works well so long as the upstream channel is relatively noise free. However, in the presence of significant noise, the CMTS cannot always correctly compute the relevant power of signals it receives. This is because the signals may be rapidly fluctuating. The instantaneous power measurements made at the X point may be arbitrarily distorted in the positive or negative direction due to the noise. In this case, continuous power adjustments may be requested by the CMTS. If power level adjustments are made continuously, the head-end may wrongly get the conclusion that a modem cannot be ranged properly and disconnect the modem, even if there is no problem except a high but tolerable noise level.
What is needed therefore is an improved technique for controlling cable modem power levels in the presence of upstream noise.